2010 Volkswagen GTI: The Effect Of Octane On Its Power

A few weeks back, one of our editors filled our longterm 2010 Volkswagen GTI's tank with 87 octane. Whether VW intended for this to happen or not, reality has a way of rendering such things moot.

Then we had a bright -- if obvious -- idea: let's do a dyno test to quantify just how much power this little hatch loses as a result of the lower octane.

It went down like this: We ran that tank of 87-ish octane down to nearly empty and refilled with 87 to ensure that the only thing in the tank was 87 octane. That, and it would give the electronic German brain on board adequate opportunity to recalibrate itself for the lower octane.

Then we dynoed it, performing as many runs as necessary to achieve a stable and consistent result.

Afterwards, we ran that tank down and refilled with 91 octane (that's the highest we get for premium fuel here in California), ran that tank down and refilled again with 91. Same logic as before.

We hit the dyno rollers again a few days later. Here's the result:

Peak Power (hp) Peak Torque (lb-ft)

91 octane 207 219

87 octane 203 216

Peak numbers don't tell the whole story, as the largest observed differences at any given engine speed were 10 hp and 11 lb-ft.

In summary, 87 octane hasn't had a tremendous effect on our GTI's ability to hustle. Surprised? I was. I expected a larger difference than this.

It's likely that although the output is similar, exhaust gas temperatures may not be, as less ignition timing can be run with 87 octane. As a result, there might be:

less altitude margin with 87 octane, and a greater power difference might be observed at high elevation. Sorry, I don't plan on repeating this test in Denver.

Note that modern engine controllers are quite adept at monitoring knock activity and adjusting accordingly. I heard not a single ping when running the snot out of this car on the dyno in either test. If you put 87 octane in your twenty year-old turbo car and gave it the wood, you might make engine soup.

Speaking of dynos, we dyno-tested our GTI on 91 octane last year, so why re-test it? I wanted to ensure similar weather conditions between the two octane dyno tests to eliminate that as a variable to the extent that I could. There are certain things that are still beyond my control, and weather is one of them. Across these two dyno test days, the temperature was within 9 degrees and ambient pressure was within 0.04 in Hg. That's about as close as anyone could ask for.

Weather has a different effect on modern turbocharged cars than it does on supercharged or normally aspirated cars. Here's why: power depends largely on the airflow rate into the engine. Turbo cars have the unique ability to regulate that airflow independently of ambient air density -- unlike non-turbo engines, turbo'd ones can actively manipulate boost pressure so that the same amount of flow enters the engine over a very broad range of ambient conditions. There are limits to this, of course, but the upshot is that turbo engines don't suffer nearly the power loss of a non-turbo engine when, say, climbing a long grade.

Temperature, however, does affect how much timing a turbo engine can run (due to knock), so in that respect weather has an indirect affect on the output of a turbo engine. The thing is, weather correction factors only account for density changes, which is why SAE does not apply weather correction to turbocharged cars, and neither do we.